Shorebirds forage disproportionately in horseshoe crab nest depressions

Horseshoe Crab (Limulus polyphemus) eggs are an important shorebird food during the Delaware Bay spring stopover, and shorebird management plans aim to increase and monitor this resource. If shorebirds focus their foraging on Horseshoe Crab nesting depressions they may find richer food supplies than if they forage randomly on the beach. The amount of shorebird sign in quadrats centered on Horseshoe Crab nest depressions was compared with the amount of sign in paired beach areas with no Horseshoe Crab nests. Horseshoe Crab nest depressions had more pecks, probes, digit marks, Ruddy Turnstone excavations, Horseshoe Crab eggs and a greater coverage by shorebird sign than paired beach plots with no Horseshoe Crab nests. Foraging disproportionately within Horseshoe Crab nesting depressions may facilitate the rapid mass gain needed to prepare shorebirds for their flight to the breeding grounds. Horseshoe crab egg monitoring currently estimates mean egg abundance per beach. However, birds are able to find patches with high egg densities even when foraging on beaches with lower average densities.